1. Field of the Invention
The invention relates to a measuring device for measuring the phase deviation of at least one subordinate transport unit of a synchronous signal transmitted by means of a synchronous transmission system.
2. Description of the Related Art
In a synchronous transmission system, which may be a system according to the synchronous digital hierarchy or the American SONET system (Synchronous Optical Network), it is possible to join together, split up, branch off, feed in, or bypass signal streams, as desired. For example, the plesiochronous payload signal streams coming in into a network junction of the synchronous digital hierarchy (in Europe: 2.048 Mbit/s, 34.368 Mbit/s and 139.264 Mbit/s) may be so prepared by mapping that they are always dispatched on their transmission path in a unified, 125 .mu.s long synchronous transport frame (STM-1 frame) as STM-1 signals with a bitrate of 155.52 Mbit/s. Such a network junction is also capable of receiving and processing STM-N signals (N=4, 16, . . .) arising through multiplexing of STM-1 signals.
The STM-1 signal is structured into frames and comprises besides the actual payload data of the signal also control information and stop data. An STM-1 frame consists of 270 columns and 9 lines (270 bytes per line). The lines 1 to 3 and 5 to 9, in columns 1 to 9 each time, contain the "Section Overhead" (SOH) for control and error recognition information, while the remaining space (AU payload space) contains signal data, stop data, and further control information.
Several different containers (C-4, C-3, C-2, C-12 and C-11) may be accommodated in the AU payload space. A container is the basic packaging unit for digital payload signals. For example, an administrative unit AU-4 with a container C-4 for a bitrate of 139.264 Mbit/s may be accommodated in an STM-1 frame. Furthermore, three administrative units AU-3 may be accommodated in the STM-1 frame. Of these, for example, one administrative unit AU-3 comprises a container C-3 for a bitrate of 44.736 Mbit/s. The second administrative unit AU-3 may contain, for example, seven tributary unit groups TUG-2 each with a container C-2 for a bitrate of 6.312 Mbit/s. In the third administrative unit AU-3, furthermore, seven TUG-2 each with three containers C-12 for a bitrate of 2.048 Mbit/s may be accommodated. Further transport units (VC-4, VC-3, TU-3, TU-2, TU-12 and TU-11) are formed from the containers through the addition of control information and stop information.
The transmission of a subordinate transport unit, which may be a transport unit of a plesiochronous signal (for example, 2.048 Mbit/s) or a virtual container VC-2, VC-12 or VC-11, via several synchronous digital devices leads to passage timedependent phase shifts between the STM-1 signal formed in a synchronizer with at least one subordinate transport unit and the STM-1 signal with the subordinate transport unit received in a desynchronizer. Furthermore, shifts of transport units dependent on buffer fillings arising from frequency and phase fluctuations, which shifts lead to a change in at least one pointer value of a transport unit and thus also cause stop processes, effect an additional phase shift of the subordinate transport unit. The additional phase shift which, as stated above, is indicated by a change in an AU pointer (in the case of a shift of a VC-3 or VC-4) or a TU pointer (in the case of a shift of a VC-2, VC-12 or VC-11) must be taken into account during the desynchronization.
A measuring device for a SDH system is known, for example, from the article "2,5-GBit/s-Leitungsausrustung im Projekt Berlin V" (2.5-Gbit/s Line Equipment in the Berlin V Project), ntz, vol. 44, 1991, no. 11, pp. 782-788. The block diagram of this measuring device is shown in FIG. 7 of the article. An SDH analyser (transmitter) forms an STM-1 signal which is supplied to an SDH system. In the SDH system, an STM-16 signal is formed from several STM-1 signals by a multiplexer and guided towards an optical receiver of the SDH system via an optical transmitter and a light waveguide. The received STM-1 signal is supplied to the receiver unit of the SDH analyser from the optical receiver through a demultiplexer. In addition, a frequency counter and a personal computer are present, by means of which the phase deviation between the transmitted and received STM-1 signal is to be ascertained. The STM-1 signal supplied by the SDH analyser is written into the multiplexer of the SDH system by means of a write clock signal with a first frequency. The STM-1 signal is read out from the multiplexer by means of a read clock signal with a second frequency. The conditions are exactly the reverse in the demultiplexer. The read clock signal of the multiplexer is here used as the write clock signal, and the write clock signal of the multiplexer is used as the read clock signal. It is possible with this measuring device only to ascertain the phase deviation of the virtual container VC-4 in the STM-1 signal after passage through the SDH system. A phase deviation of a subordinate transport unit passing through the SDH system cannot be measured.